<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<h1>0</h1>
<script>
  let count=0
  setInterval(f,1000)
  function f() {
    count++
    let h = document.querySelector("h1")
    h.innerText=count;
  }
  let timer = setInterval(function () {
    console.log(count)
    if (count==5){
      clearInterval(timer)
    }
  },1000)

  //开启只执行一次的定时器
  setTimeout(function () {
    console.log("时间到")
  },3000)
</script>
</body>
</html>